One in five milk samples nationwide shows traces of bird flu

Federal regulators have discovered fragments of bird flu virus in roughly 20% of retail milk samples tested in a nationally representative study, the Food and Drug Administration said in an online update on Thursday.

Samples from parts of the country that are known to have dairy herds infected with the virus were more likely to test positive, the agency said. Regulators said there is no evidence that this milk poses a danger to consumers or that live virus is present in the milk on store shelves, an assessment public health experts have agreed with.

But finding traces of the virus in such a high share of samples from around the country is the strongest signal yet that the bird flu outbreak in dairy cows is more extensive than the official tally of 33 infected herds across eight states.

“It suggests that there is a whole lot of this virus out there,” said Richard Webby, a virus and influenza expert at St. Jude Children’s Research Hospital.

Webby said he believed it was still possible to eradicate the virus, which is known as H5N1, from the nation’s dairy farms. But it will be difficult to design effective control measures without knowing the scope of the outbreak, he said.

The findings also raise questions about how the virus has evaded detection and where else it might be silently spreading. Some scientists have criticized the federal testing strategy as too limited to reveal the true extent of viral spread.

Until Wednesday, when the Department of Agriculture announced mandatory testing of dairy cows moving across state lines, testing of cows had been voluntary and primarily focused on cows with obvious symptoms.

As of Wednesday, just 23 people had been tested for the virus, while 44 people were being monitored after exposure to it, according to the Centers for Disease Control and Prevention.

A widespread outbreak in cows would pose a greater risk to farmworkers, the dairy industry and public health more broadly. Sustained spread among cows would give the virus more opportunities to acquire mutations that make it more transmissible among humans.

The FDA did not provide details Thursday regarding the number or sources of the samples.